Top dressing foliar fertilizer after water stress?

Question:  Is top dressing nutrients on wheat and/or canola effective to aid crops after high amounts of rain?

Answer Prepared by Dr. Don Flaten, Faculty of Soil Science, University of Manitoba:

With the unusually large rainfall events across much of Manitoba, some farmers are noticing yellowing in their crops and wondering if they should top up their fields with some more nitrogen fertilizer. Unfortunately, there are no easy rules to follow or large experimental datasets to refer to for making this type of decision.  Some things to consider are:

Is crop yellowing is from N deficiency or simply excess water? Applying N fertilizer onto crops that are suffering mostly from flooding stress could be a  poor  investment and be at high risk for losing large amounts of the applied N. Heavy rains and saturated soils can cause large losses of nitrogen in sandy soils due to leaching or clay soils due to denitrification, especially when the weather is warm. Medium textured soils might experience moderate losses due to both processes. The only way to tell for sure is to soil test, but that’s very difficult to do in a timely manner at this time of year, especially under conditions of excess water.

If the crop is likely to recover from the excess moisture and yield potential good, a timely application of N can increase yields dramatically. Recent research by AAFC in Eastern Saskatchewan showed excellent yield responses to midseason applications of 30-60 lbs of N as UAN liquid fertilizer (28-0-0) as late as the 5.5 leaf stage for wheat (stem elongation) and the mid-bolting stage for canola.

Concerned about volatilization? Given the warm and often windy conditions at this time of year, farmers who are concerned about volatilization losses from UAN liquid fertilizer (28-0-0) or from granular urea (46-0-0) may want to add Agrotain N-stabilizer with their fertilizer. Agrotain slows down the conversion of urea-N to ammonia in both of these types of fertilizer, helping to reduce the risk of gaseous losses due to volatilization.

Extra Notes on Midseason Application of N Fertilizer: Surface applications of fertilizer are often less efficient than in-soil banded or incorporated fertilizers. Lack of incorporation will increase the risk of volatilization and immobilization losses. Using a dribble-band and not a spray or broadcast method (i.e. not the way you apply a herbicide) will reduce contact between the fertilizer and crop residue, reducing volatilization and immobilization losses from surface applications.  In several field studies in Manitoba, surface dribble-banded applications of UAN were nearly as effective as in-soil banded applications.

References and Further Reading:

Spring Options for Nitrogen Fertilization, presented at the 2004 Manitoba Agronomist’s Conference by Dr. Cynthia Grant)

Lafond, G. P., Brandt, S. A., Irvine, B., May, W. E. and Holzapfel, C. B. 2008. Reducing the risks of in-crop nitrogen fertilizer applications in spring wheat and canola. Can. J. Plant Sci. 88: 907-919.

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